Neuromorphic device

ABSTRACT

A neuromorphic device includes a substrate; a first electrode and a second electrode that are disposed over the substrate, extend in a first direction, and are spaced apart in a second direction; a stack structure between the first electrode and the second electrode, which includes reactive metal layers alternately stacked with one or more insulating layers; an oxygen-containing layer between the first electrode and the stack structure, which includes oxygen ions; and an oxygen diffusion-retarding layer between the stack structure and the oxygen-containing layer. The first direction is perpendicular to a top surface of the substrate, and the second direction is parallel to the top surface of the substrate. Each reactive metal layer may react with the oxygen ions to form a dielectric oxide layer. The oxygen diffusion-retarding layer interferes with a movement of the oxygen ions. A thickness of the oxygen diffusion-retarding layer varies along the first direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 62/273,255, filed on Dec. 30, 2015, and Korean PatentApplication No. 10-2016-0164970, filed on Dec. 6, 2016, which areincorporated herein by reference in their entirety.

BACKGROUND

1. Field

Exemplary embodiments of the present disclosure relate to neuromorphicdevices that mimic a human nervous system, and their applications.

2. Description of the Related Art

Recently, as electronic appliances trend toward miniaturization, lowpower consumption, high performance, multi-functionality, and so on,technology capable of efficiently processing large-volume informationhas been demanded. In particular, neuromorphic technology for mimickingneuro-biological architectures present in a human nervous system hasreceived much attention to implement the technology of efficientlyprocessing large-volume information. The human nervous system includesseveral thousand billions of neurons and synapses serving as junctionsbetween the respective neurons. In the neuromorphic technology, neuroncircuits and synapse circuits, which correspond to neurons and synapsesof the human nervous system, respectively, are designed to realizeneuromorphic devices. The neuromorphic devices may be used in variousapplications including data classification, pattern recognition, and thelike.

SUMMARY

Embodiments of the present disclosure are directed to neuromorphicdevice including a synapse having enhanced symmetry and linearity.

In accordance with an embodiment, a neuromorphic device includes: asubstrate; a first electrode and a second electrode that are disposedover the substrate, the first and second electrodes extending in a firstdirection and being spaced apart from each other in a second direction,the first direction being perpendicular to a top surface of thesubstrate, the second direction being parallel to the top surface of thesubstrate; a stack structure disposed between the first electrode andthe second electrode, the stack structure including a plurality ofreactive metal layers which are alternately stacked with one or moreinsulating layers, wherein each of the reactive metal layers is capableof reacting with oxygen ions to form a dielectric oxide layer; anoxygen-containing layer disposed between the first electrode and thestack structure, the oxygen-containing layer including the oxygen ions;and an oxygen diffusion-retarding layer disposed between the stackstructure and the oxygen-containing layer, the oxygendiffusion-retarding layer interfering with a movement of the oxygen ionsfrom the oxygen-containing layer to the reactive metal layers, andwherein a thickness of the oxygen diffusion-retarding layer varies alongthe first direction.

In the above embodiment, the thickness of the oxygen diffusion-retardinglayer increases from a top of the oxygen diffusion-retarding layer to abottom of the oxygen diffusion-retarding layer. The thickness of theoxygen diffusion-retarding layer decreases from a top of the oxygendiffusion-retarding layer to a bottom of the oxygen diffusion-retardinglayer. The second electrode includes a plurality of second electrodes,which are spaced apart from each other, and wherein the stack structureincludes a plurality of stack structures, which are spaced apart fromeach other, each of the plurality of stack structures corresponding to arespective one of the plurality of the second electrodes. Theneuromorphic device further comprises: an interlayer insulating layerdisposed between two adjacent second electrodes and between two adjacentstack structures corresponding to the two adjacent second electrodes.The neuromorphic device further comprises: a slit penetrating throughthe interlayer insulating layer and extending in a direction crossing aregion between the two adjacent second electrodes and between the twoadjacent stack structures, the slit extending from the first electrode.The neuromorphic device further comprises: a filling layer disposed inthe slit, the filling layer including the same material as at least oneof the oxygen-containing layer and the oxygen diffusion-retarding layer.Thicknesses of the plurality of reactive metal layers are different fromeach other. Thicknesses of the plurality of reactive metal layersincrease along a direction from a top of the stack structure to a bottomof the stack structure. Thicknesses of the reactive metal layersdecrease along a direction from a top of the stack structure to a bottomof the stack structure. The oxygen-containing layer is disposed againsta side surface of the first electrode, the oxygen-containing layerencircling the first electrode. The oxygen diffusion-retarding layer isdisposed against a side surface of the oxygen-containing layer, theoxygen diffusion-retarding layer encircling the oxygen-containing layer.A reset operation is performed when a reset voltage having a firstpolarity is applied through the first electrode and the secondelectrode, the reset operation including forming or thickening thedielectric oxide layer in at least one of the plurality of reactivemetal layers at an interface with the oxygen diffusion-retarding layer,and wherein a set operation is performed when a set voltage having asecond polarity opposite to the first polarity is applied through thefirst electrode and the second electrode, the set operation includingdisappearing or thinning the dielectric oxide layer. The dielectricoxide layer is formed the fastest in a first reactive metal layer of theplurality of reactive metal layers, the first reactive metal layer beingseparated from the oxygen-containing layer by a thinnest region of theoxygen diffusion-retarding layer. Thicknesses of the plurality ofreactive metal layers are different from each other, and the dielectricoxide layer is formed the fastest in a first reactive metal layer of theplurality of reactive metal layers, the first reactive metal layerhaving the smallest thickness of the plurality of reactive metal layers.The dielectric oxide layer includes one or more dielectric oxide layers,and wherein an electrical conductivity decreases as a number of the oneor more dielectric oxide layers increases or a thickness of each the oneor more dielectric oxide layers increases, and the electricalconductivity increases as the number of the one or more dielectric oxidelayers decreases or the thickness of each of the one or more dielectricoxide layers decreases. The dielectric oxide layer includes one or moredielectric oxide layers, and wherein, during the reset operation, anumber of the one or more dielectric oxide layers increases or athickness of each of the one or more dielectric oxide layers increaseswhen a number of pulses of the reset voltage applied through the firstelectrode and the second electrode increases, and wherein, during theset operation, the number of the one or more dielectric oxide layersdecreases or a thickness of each of the one or more dielectric oxidelayers decreases when a number of pulses of the set voltage appliedthrough the first electrode and the second electrode increases. Duringthe reset operation, the pulses of the reset voltage have a constantwidth and magnitude, and wherein, during the set operation, the pulsesof the set voltage have a constant width and magnitude. The oxygendiffusion-retarding layer has a thickness that incompletely blocks themovement of the oxygen ions. The oxygen diffusion-retarding layercomprises a dielectric material, a semiconductor material, or acombination of the dielectric material and the semiconductor material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a neuromorphic device according to an embodiment.

FIGS. 2A to 2D illustrate characteristics of a synapse of theneuromorphic device shown in FIG. 1.

FIG. 3A is a cross-sectional view illustrating a synapse of acomparative example.

FIGS. 3B and 3C illustrate characteristics of the synapse shown in FIG.3A.

FIGS. 4A and 4B are views illustrating a neuromorphic device accordingto an embodiment.

FIGS. 5A to 5C are cross-sectional views illustrating a reset operationof a synapse of FIGS. 4A and 4B.

FIG. 6 is a planar view illustrating a neuromorphic device according toanother embodiment.

FIGS. 7A to 10B are views illustrating a method for fabricating aneuromorphic device according to an embodiment.

FIG. 11 is a cross-sectional view illustrating a neuromorphic deviceaccording to yet another embodiment.

FIG. 12 is a cross-sectional view illustrating a neuromorphic deviceaccording to yet another embodiment.

FIG. 13 is a graph illustrating a current through a synapse according toa size of a contact area and/or facing area between a reactive metallayer and an oxygen-containing layer in the synapse.

FIG. 14 is a cross-sectional view illustrating a neuromorphic deviceaccording to yet another embodiment.

FIG. 15 shows a pattern recognition system according to an embodiment.

DETAILED DESCRIPTION

Various examples and implementations of the disclosed technology aredescribed below in detail with reference to the accompanying drawings.

The drawings may not be necessarily to scale and in some instances,proportions of at least some of structures in the drawings may have beenexaggerated in order to clearly illustrate certain features of thedescribed examples or implementations. In presenting a specific examplein a drawing or description having two or more layers in a multi-layerstructure, the relative positioning relationship of such layers or thesequence of arranging the layers as shown reflects a particularimplementation for the described or illustrated example and a differentrelative positioning relationship or sequence of arranging the layersmay be possible. In addition, a described or illustrated example of amulti-layer structure may not reflect all layers present in thatparticular multilayer structure (e.g., one or more additional layers maybe present between two illustrated layers). As a specific example, whena first layer in a described or illustrated multi-layer structure isreferred to as being “on” or “over” a second layer or “on” or “over” asubstrate, the first layer may be directly formed on the second layer orthe substrate but may also represent a structure where one or more otherintermediate layers may exist between the first layer and the secondlayer or the substrate.

FIG. 1 illustrates a neuromorphic device according to an embodiment.

Referring to FIG. 1, the neuromorphic device according to the embodimentmay include a plurality of pre-synaptic neurons 10, a plurality ofpost-synaptic neurons 20, and a plurality of synapses 30 that provideconnections between the plurality of pre-synaptic neurons 10 and theplurality of post-synaptic neurons 20.

For illustrative convenience, the neuromorphic device of the embodimentshown in FIG. 1 includes four pre-synaptic neurons 10, fourpost-synaptic neurons 20, and sixteen synapses 30, but the numbers ofpre-synaptic neurons, post-synaptic neurons, and synapses in theneuromorphic device may be changed in various ways. If the number ofpre-synaptic neurons 10 is N and the number of post-synaptic neurons 20is M, N*M synapses 30 may be arranged in a matrix form, wherein N and Mare natural numbers equal to or greater than 2, and N and M may or maynot be equal to each other.

For this arrangement shown in FIG. 1, the neuromorphic device mayfurther include a plurality of first lines 12 and a plurality of secondlines 22. The plurality of first lines 12 may be coupled to theplurality of pre-synaptic neurons 10, respectively, and may extend in afirst direction, for example, a horizontal direction with respect to theorientation of FIG. 1. The plurality of second lines 22 may be coupledto the plurality of post-synaptic neurons 20, respectively, and mayextend in a second direction crossing the first direction, for example,a vertical direction with respect to the orientation of FIG. 1.Hereinafter, for convenience of explanation, the first lines 12 will bereferred to as row lines, and the second lines 22 will be referred to ascolumn lines. The plurality of synapses 30 may be disposed atintersections between the row lines 12 and the column lines 22. Each ofthe synapses 30 may couple a corresponding one of the row lines 12 to acorresponding one of the column lines 22. In other words, the pluralityof synapses 30 may be disposed in regions where the row lines 12 overlapwith the column lines 22. That is, each of the synapses 30 may bedisposed in an intersection region between the corresponding row line 15and the corresponding column line 25.

The pre-synaptic neurons 10 may generate a signal (e.g., a signalcorresponding to certain data) and transmit the generated signal to therow lines 12. The post-synaptic neurons 20 may receive, through thecolumn lines 22, a synaptic signal corresponding to the signal that wasgenerated by the pre-synaptic neurons 10 and has passed through thesynapses 30, and may process the received signal.

The row lines 12 may correspond to axons of the pre-synaptic neurons 10,and the column lines 22 may correspond to dendrites of the post-synapticneurons 20. However, whether a neuron of interest is a pre-synapticneuron or a post-synaptic neuron may be determined by its relationshipwith another neuron. For example, a neuron receiving a synaptic signalfrom another neuron may function as a post-synaptic neuron. Similarly, aneuron transmitting a signal to another neuron may function as apre-synaptic neuron. The pre-synaptic neurons 10 and the post-synapticneurons 20 may be implemented using various circuits such ascomplementary metal-oxide-semiconductor (CMOS) circuits.

The pre-synaptic neurons 10 and the post-synaptic neurons 20 areelectrically coupled by the synapses 30. Herein, the synapse 30 is anelement that has an electrical conductance or a weight changingaccording to an electrical pulse (e.g., a voltage or current) applied tothe synapse 30.

Each of the synapses 30 may include a variable resistance element. Thevariable resistance element is an element capable of switching betweendifferent resistance states according to a voltage or current that isapplied to both ends thereof. The variable resistance element may have asingle-layered structure, or may have a multi-layered structure thatincludes any of various materials. The single-layered structure may havea plurality of resistance states by itself. The multi-layered structuremay have a plurality of resistance states by combinations of the variousmaterials. The various materials may include any of metal oxides such astransition metal oxides or perovskite-based materials, phase-changematerials such as chalcogenide-based materials, ferroelectric materials,ferromagnetic materials, and the like.

An operation in which the variable resistance element of the synapse 30switches from a high-resistance state to a low-resistance state may bereferred to as a set operation, and an operation in which the variableresistance element of the synapse 30 switches from the low-resistancestate to the high-resistance state may be referred to as a resetoperation.

However, unlike variable resistance elements that are used in memorydevices such as RRAM, PRAM, FRAM, and MRAM devices, a resistance valueof the synapse 30 in the neuromorphic device does not change abruptly inthe set operation and the reset operation. Instead, the synapse 30exhibits an analog behavior in which electrical conductivity of thesynapse 30 gradually changes according to the number and/or magnitude ofelectrical pulses applied to the synapse 30 during the set operation andthe reset operation. Thus, the synapse 30 may have variouscharacteristics distinguishable from those of a variable resistanceelement of another type of memory device, because the characteristics ofthe synapse 30 in the neuromorphic device differ from characteristicsrequired for a variable resistance element of another type of memorydevice.

On the other hand, another type of memory device preferably uses avariable resistance element that maintain its electrical conductivitybefore a set operation or a reset operation is performed, even ifelectrical pulses are repeatedly applied to the variable resistanceelement. Accordingly, the variable resistance element may storedifferent data by having clearly distinguished low-resistance andhigh-resistance states.

Meanwhile, an example of a learning operation of the neuromorphic deviceof FIG. 1 will be described. For convenience of explanation, the fourrow lines 12 may be sequentially referred to as a first row line 12A, asecond row line 12B, a third row line 12C, and a fourth row line 12D, asillustrated from the top to the bottom of FIG. 1. The four column lines22 may be sequentially referred to as a first column line 22A, a secondcolumn line 22B, a third column line 22C, and a fourth column line 22D,as illustrated from the left to the right of FIG. 1.

Each of the synapses 30 may undergo a set operation by switching from ahigh-resistance state to a low-resistance state, and may undergo a resetoperation by switching from a low-resistance state to a high-resistancestate. An electrical conductivity of each of the synapses 30 increasesduring a potentiation operation, and decreases during a depressionoperation.

In an initial stage of the learning operation, each of the synapses 30may be in a high-resistance state. If at least one of the synapses 30 isin a low-resistance state, an initialization operation for changing thelow-resistance state of the synapses 30 to the high-resistance state maybe performed in order to bring the synapses 30 to the initial stage.

Each of the synapses 30 may have a certain critical value. Morespecifically, if a voltage or current lower than the certain criticalvalue is applied to the synapse 30, the electrical conductivity of thesynapse 30 may not change, and if a voltage or current equal to orhigher than the certain critical value is applied to the synapse 30, theelectrical conductivity of the synapse 30 may change.

In the initial stage, in order to perform an operation for learningcertain data in any column line 22, an input signal corresponding to thecertain data may be input to the row lines 12 so that an electricalpulse is selectively applied to each of the row lines 12 according tothe certain data. The input signal may be input by applying electricalpulses to row lines 12 corresponding to ‘1’ in the certain data, and notto row lines 12 corresponding to ‘0’ in the certain data. For example,if an input signal corresponding to certain data ‘0011’ is input to therow lines 12 of FIG. 1, an electrical pulse may not be applied to thefirst and second row lines 12A and 12B, and may be applied to the thirdand fourth row lines 12C and 12D.

While the input signal is being input, the column lines 22 may beselectively driven at a suitable voltage or current for the learningoperation.

As an example, if a column line 22 to learn specific data ispredetermined, the predetermined column line 22 may be driven such thatsynapses 30 located at intersections between the row lines 12corresponding to ‘1’ and the predetermined column 22 may receive avoltage equal to or higher than a set voltage. The set voltage may be avoltage required for the set operation. Simultaneously, the remainingcolumn lines 22, which are not the predetermined column line 22, may bedriven such that the remaining synapses 30 may receive a voltage lowerthan the set voltage. Referring to FIG. 1, the remaining synapses aresynapses other than the synapses 30 located at the intersections betweenthe row lines 12 corresponding to ‘1’ and the predetermined column lines22.

For example, if the magnitude of the set voltage is Vset and the thirdcolumn line 22C is predetermined as a column line to learn the certaindata ‘0011,’ the magnitude of the electrical pulse that is applied toeach of the third and fourth row lines 12C and 12D may be equal to orhigher than Vset, and a voltage that is applied to the third column line22C may be 0 V, such that first and second synapses 30A and 30B locatedat intersections between the third column line 22C and the third andfourth row lines 12C and 12D receive a voltage equal to or higher thanVset. Thus, the first and second synapses 30A and 30B may switch fromthe high-resistance state to a low-resistance state. That is, each ofthe first and second synapses 30A and 30B may undergo the set operation.

The electrical conductivity of the first and second synapses 30A and 30Bin the low-resistance state may gradually increase as the number ofelectrical pulses applied to the first and second synapses 30A and 30Bincreases. That is, each of the first and second synapses 30A and 30Bmay undergo a potentiation operation.

On the other hand, a voltage applied to the remaining column lines 22,that is, to the first, second, and fourth column lines 22A, 22B, and22D, may have a magnitude between 0V and Vset such that the remainingsynapses 30 receive a lower voltage than Vset. For example, the voltageapplied to the remaining column lines 22 may be equal to ½ Vset. Thus,resistance states of the remaining synapses 30, which are synapses 30other than the first and second synapses 30A and 30B, may not change.

If the row lines 12 and the column lines 22 are driven in theabove-described manner, the electrical conductivity of synapses 30 thatreceive electrical pulses may gradually increase, and thus a currentflowing through the synapses 30 that receive the electrical pulses mayincrease. For example, the electrical conductivity of the synapses 30Aand 30B may increase when the synapses 30A and 30B receive theelectrical pulses from the third and fourth row lines 12C and 12D, and acurrent flowing to the third column line 22C through the synapses 30Aand 30B may increase. When the current flowing to the third column line22C is measured and the measured current reaches a certain criticalcurrent, the third column line 22C may be a ‘column line that has leanedspecific data,’ for example, a column line that has learned the certaindata ‘0011.’

As another example, a column line 22 to learn specific data may not bepredetermined. In this case, a current flowing to each of the columnlines 22 is measured while electrical pulses corresponding to thespecific data are applied to the row lines 12. Based on the measurementresults, a column line, e.g., the first column line 22A, which reachedthe certain critical current before the other column lines, e.g., thesecond to fourth column lines 22B to 22D, may be determined to be acolumn line that has learned the specific data.

In the above-described manner, the other column lines may learndifferent data in other learning operations.

Meanwhile, although the learning operation described above includes onlythe set operation of changing the resistance state of the synapses 30from a high-resistance state to a low-resistance state and thepotentiation operation of increasing the electrical conductivity of thesynapses 30, the learning operation may also include the reset operationof changing the resistance state of the synapses 30 from thelow-resistance state to the high-resistance state and the depressionoperation for reducing the electrical conductivity of the synapses 30.

For example, the polarity of pulses that are applied in the setoperation of the synapses 30 and the potentiation operation ofincreasing the electrical conductivity of the synapses 30 may beopposite to the polarity of pulses that are applied in the resetoperation of the synapses 30 and the depression operation of reducingthe electrical conductivity of the synapses 30.

Hereinafter, characteristics of a synapse suitable for a neuromorphicdevice will be described in detail with reference to FIGS. 2A to 2D.

FIGS. 2A to 2D illustrate characteristics associated with each of thesynapses 30 shown in FIG. 1.

Specifically, FIGS. 2A and 2B illustrate the electrical conductivity Gof a synapse 30 according to the number of electrical pulses that areapplied to the synapse 30. FIG. 2C shows a change in weight ΔW of thesynapse 30 with a change in a resistance value R or electricalconductivity G of the synapse 30. FIG. 2D shows the change in weight ΔWof the synapse 30 according to a magnitude of a voltage V that isapplied to the synapse 30.

Referring to FIGS. 2A and 2B, if first-polarity voltage pulses (e.g.,negative voltage pulses) with a voltage higher than a certain criticalvalue are repeatedly applied to the synapse 30 that is in alow-resistance state, the electrical conductivity G of the synapse 30may gradually increase. A direction in which the electrical conductivityG of the synapse 30 increases may be referred to as a G+ direction or apotentiation direction.

If second-polarity voltage pulses (e.g., positive voltage pulse) with avoltage equal to or higher than the reset voltage are applied to thesynapse 30, the reset operation may be performed, such that theresistance state of the synapse 30 changes to a high-resistance state.

If second-polarity voltage pulses are repeatedly applied to the synapse30 when the synapse 30 is in the high-resistance state, the electricalconductivity G of the synapse 30 may gradually decrease. A direction inwhich the electrical conductivity G of the synapse 30 decreases may bereferred to as a G− direction or a depression direction.

If first-polarity voltage pulses with a voltage equal to or higher thanthe set voltage are applied again to this synapse 30, the set operationmay be performed so that the resistance state of the synapse 30 changesagain to the low-resistance state.

Herein, it is preferable that, if the magnitude and width of pluses areconstant, a change in the electrical conductivity G of the synapse 30 inthe potentiation operation be substantially symmetric with a change inthe electrical conductivity G of the synapse 30 in the depressionoperation, and a rate of change in the electrical conductivity G issubstantially constant in each of the potentiation operation and thedepression operation. In other words, it is preferable that theelectrical conductivity G of the synapse 30 has linearity and symmetryin the potentiation operation and the depression operation, such that aresistance value of the synapse 30 does not abruptly change in the setoperation or the reset operation. If the magnitude and/or width of thepulses must be varied to secure the linearity and symmetry of thesynapse 30, it may be necessary to implement additional circuits in theneuromorphic device to generate various pulses. The addition ofadditional circuits may be disadvantageous in terms of area or power.Therefore, it is preferable to control the pulses to have the constantmagnitude and width while driving the synapse 30.

The linearity and symmetry of the electrical conductivity G of thesynapse 30 in the potentiation operation and the depression operationmay be observed in both the case in which a rate of change in the weightof the synapse 30 is small (see ΔW1 in FIG. 2B) and the case in whichthe rate of change in the weight of the synapse 30 is great (see ΔW2 inFIG. 2B). However, if the magnitude or width of pulses is notsufficiently large, the electrical conductivity G of the synapse 30 maynot change regardless of the number of the pulses being applied to thesynapse 30.

Referring to FIG. 2C, it is preferable that the rate of change in theweight ΔW of the synapse 30 be substantially constant regardless of apresent state of the synapse 30, that is, the present resistance value Ror present electrical conductivity G of the synapse 30.

Referring to FIG. 2D, when a voltage equal to or lower than a certaincritical value is applied, for example, V₃, the weight W and/orelectrical conductivity G of the synapse 30 may not change. Namely, therate of change in the weight ΔW of the synapse 30 may be 0. On the otherhand, at a voltage higher than the certain critical value V₃, forexample, a voltage equal to or higher than V₄, the rate of change in theweight ΔW of the synapse 30 may increase. Herein, the rate of change inthe weight ΔW of the synapse 30 may increase substantially in proportionto the magnitude of the voltage applied to the synapse 30.

In summary, it is preferable that the electrical conductivity G of thesynapse 30 of the neuromorphic device increase or decrease substantiallyin proportion to the number of electrical pulses being applied to thesynapse 30, regardless of the present state of the synapse 30. It isalso preferable that a change in the electrical conductivity G of thesynapse 30 in the potentiation operation be symmetric with a change inthe electrical conductivity G of the synapse 30 in the depressionoperation. Herein, it is preferable that the change in the electricalconductivity G of the synapse 30 occur only at a voltage equal to orhigher than the certain critical value. As the characteristics of thesynapse 30 are closer to the above-described characteristics, thelearning and recognition accuracy of the neuromorphic device mayincrease, and thus operating characteristics of the neuromorphic devicemay be improved.

Embodiments of the present disclosure are directed to a synapse capableof satisfying the above-described characteristics shown in FIGS. 2A to2D to the maximum possible extent. Prior to the description of theembodiments, a synapse of a comparative example will be described.

FIG. 3A is a cross-sectional view illustrating a synapse of acomparative example, and FIGS. 3B and 3C illustrate characteristics ofthe synapse shown in FIG. 3A.

Referring to FIG. 3A, the synapse 100 of the comparative example mayinclude a first electrode 110, a second electrode 140, anoxygen-containing layer 120 disposed between the first electrode 110 andthe second electrode 140, and a reactive metal layer 130 interposedbetween the oxygen-containing layer 120 and the second electrode 140.The reactive metal layer 130 is capable of reacting with oxygen ions ofthe oxygen-containing layer 120.

The first and second electrodes 110 and 140 may be disposed at two endsof the synapse 100 to which a voltage or current is applied, and may beformed of any of various electrically conductive materials such asmetals or metal nitrides. The first electrode 110 may be connected toone of a corresponding row line 12 and a corresponding column line 22shown in FIG. 1, and the second electrode 140 may be connected to theother one of the corresponding row line 12 and the corresponding columnline 22, whereby the synapse 100 may be driven by electrical pulses. Atleast one of the first and second electrodes 110 and 140 may be omitted,such that the row line 12 or the column line 22 that is supposed to becoupled to the omitted electrode can function as the omitted electrode.

The oxygen-containing layer 120 is a layer containing oxygen ions, andmay include any of various metal oxides, for example, oxides oftransition metals such as Ti, Ni, Al, Nb, Hf, and V; perovskite-basedoxides such as Pr_(1-x)Ca_(x)MnO₃ (PCMO) and La_(1-x)Sr_(x)MnO₃ (LCMO),and the like.

The reactive metal layer 130 is a layer capable of reacting with oxygenions to form a dielectric oxide, and may include a metal such as Al, Ti,Ta, or Mo, or a nitride of the metal.

In an initial stage, the synapse 100 may be in a relatively lowresistance state. Thus, to perform an operation of a neuromorphicdevice, an initialization operation for changing the synapse 100 fromthe low-resistance state to a high-resistance state may be required.

If voltage pulses with a certain polarity are applied through the firstand second electrodes 110 and 140 to the synapse 100 when the synapse100 in the low-resistance state, oxygen ions in the oxygen-containinglayer 120 may move toward the reactive metal layer 130 and then reactwith a metal included in the reactive metal layer 130, thereby forming adielectric oxide layer at an interface between the oxygen-containinglayer 120 and the reactive metal layer 130. The dielectric oxide layermay include an oxide of the metal included in the reactive metal layer130. As a result, the synapse 100 may undergo the reset operation andthe resistance state of the synapse 100 may be changed to ahigh-resistance state. As the number of voltage pulses applied to thesynapse 100 increases, a thickness of the dielectric oxide layer mayincrease, and thus the synapse 100 may undergo the depression operationsuch that the electrical conductivity of the synapse 100 mayprogressively decrease.

In contrast, if voltage pulses with a polarity opposite to the certainpolarity are applied to the synapse 100 when the synapse 100 in thehigh-resistance state, oxygen ions may move in a direction from thereactive metal layer 130 toward the oxygen-containing layer 120, andthus the thickness of the dielectric oxide layer may decrease or thedielectric oxide layer may disappear. As a result, the synapse 100 mayundergo the set operation and the resistance state of the synapse 100may be changed to the low-resistance state. As the number of voltagepulses applied to the synapse 100 increases, the thickness of thedielectric oxide layer may decrease, and the synapse 100 may undergo thepotentiation operation such that the electrical conductivity of thesynapse 100 may progressively increase.

As described above, as the thickness of the dielectric oxide layerprogressively increases or decreases by the voltage pulses applied tothe synapse 100, the resistance state of the synapse 100 switchesbetween the high-resistance state and the low-resistance state. Thus,the synapse 100 may have an analog behavior, such that the electricalconductivity of the synapse 100 in each of the high-resistance state andthe low-resistance state progressively changes. However, this does notsatisfy the characteristics described above with reference to FIGS. 2Ato 2D. The characteristics of the synapse 100 will be described indetail with reference to FIGS. 3B and 3C.

Referring to FIG. 3B, if first-polarity voltage pulses are applied tothe synapse 100 that is in the low-resistance state, the synapse 100 mayundergo the potentiation operation and the electrical conductivity G ofthe synapse 100 may progressively increase as the number of the voltagepulses increases. However, a rate of increase in the electricalconductivity G is very high in an initial stage of the set operation andgradually decreases as the potentiation operation progresses. Thus,there is a problem in that the linearity of the synapse 100 is notsatisfied.

In addition, if second-polarity voltage pulses with a voltage equal toor higher than a reset voltage are applied to the synapse 100 that isthe low-resistance state, the reset operation may be performed such thatthe resistance state of the synapse 100 changes to the high-resistancestate. With an increase in the number of second-polarity voltage pulsesapplied to the synapse 100 in the high-resistance state, the synapse 100may undergo the depression operation and the electrical conductivity Gof the synapse 100 may progressively decrease. However, an abruptdecrease in the electrical conductivity G may occur in the reset anddepression operations. In addition, the rate of decrease in theelectrical conductivity G is very high in an initial stage of the resetoperation and gradually decreases as the depression operationprogresses. The degree of decrease in the electrical conductivity G inthe initial stage of the reset operation may be much larger than thedegree of increase in the electrical conductivity G in the initial stageof the set operation. Thus, as shown in FIG. 3B, there is a problem inthat the linearity and symmetry of the synapse 100 are not satisfied.

Referring to FIG. 3C, a rate of change in weight ΔW of the synapse 100is not constant according to a current resistance R of the synapse 100.In the set operation, if the present resistance value R of the synapse100 is relatively high (e.g., R₅ or R₆), the rate of change in theweight ΔW of the synapse 100 may increase in the G+ direction. In otherwords, in the initial stage of the set operation when the synapse 100has the relatively high resistance, the rate of change in the electricalconductivity G of the synapse 100 may be high. In contrast, in the resetoperation when the present resistance value R of the synapse 100 isrelatively low (e.g., R₁), the rate of change in the weight ΔW of thesynapse 100 may increase in the G− direction. In other words, in theinitial stage of the reset operation when the synapse 100 has therelatively low resistance, the rate of change in the electricalconductivity G of the synapse 100 may be high. This suggests that thelinearity of the synapse 100 is not satisfied.

Additionally, in the initial stage, the rate of change in the weight ΔWin the G− direction is higher than the rate of change in the weight ΔWin the G+ direction, as shown in FIG. 3C. This indicates that thesymmetry of the synapse 100 is not satisfied.

The reason why the above-described problems arise are that the rate ofchange in the resistance value R of the synapse 100 in each of theinitial stages of the set and reset operations is high, and that thespeed of the reset operation in which the dielectric oxide layer isformed is much higher than the speed of the set operation in which thedielectric oxide layer disappears.

Embodiments of the present disclosure are directed to synapses capableof overcoming the problems of the comparative example.

FIGS. 4A and 4B are views illustrating a neuromorphic device accordingto an embodiment. FIG. 4A shows a planar view of the neuromorphicdevice, and FIG. 4B shows a cross-sectional view of the neuromorphicdevice, taken along a line A-A′ of FIG. 4A.

Referring to FIGS. 4A and 4B, the neuromorphic device may include asubstrate 200, a first electrode 210, a second electrode 240 disposedover the substrate 200, a stack structure ST disposed between the firstelectrode 210 and the second electrode 240, an oxygen-containing layer220, and an oxygen diffusion-retarding layer 250 disposed between thestack structure ST and the first electrode 210. The first and secondelectrodes 210 and 240 may each extend in a direction substantiallyperpendicular to a top surface of the substrate 200, and may be spacedapart from each other in a horizontal direction parallel to the topsurface of the substrate 300. The stack structure ST may include aplurality of reactive metal layers 230 and a plurality of insulatinglayers 235, which are alternately stacked over the substrate 200. Theoxygen diffusion-retarding layer 250 may encircle a sidewall of thefirst electrode 210, and may extend in the direction substantiallyperpendicular to the top surface of the substrate 200. Here, the oxygendiffusion-retarding layer 250 is disposed between the stack structure STand the oxygen-containing layer 220. The oxygen-containing layer 220 isdisposed between the first electrode 210 and the oxygendiffusion-retarding layer 250.

The substrate 200 may include a lower structure (not shown), which maybe used in the neuromorphic device. The lower structure may include aline, a neuron circuit, or the like, which is coupled to the firstelectrode 210 and/or the second electrode 240.

The first and second electrodes 210 and 240 may be disposed at two endsof a synapse SN and may include any of various conductive materials suchas a metal, a metal nitride, and the like. A voltage or current may beapplied to each of the first and second electrodes 210 and 240. Thefirst electrode 210 may be connected to any one of the row lines 12shown in FIG. 1, and the second electrode 240 may be connected to anyone of the column lines 22 shown in FIG. 1, or vice versa. The synapseSN may be driven by electrical pulses applied through the first andsecond electrodes 210 and 240.

In an embodiment, one of the row lines 12 and/or one of the column lines22 of FIG. 1 may be disposed in the substrate 200, may be respectivelycoupled to lower ends of the first electrode 210 and/or the secondelectrode 240, and may respectively drive the first electrode 210 and/orthe second electrode 240. Alternatively, in another embodiment, the rowline 12 and/or the column line 22 of FIG. 1 may be disposed over thefirst electrode 210 and/or the second electrode 240, and may berespectively coupled to upper ends of the first electrode 210 and/or thesecond electrode 240.

The first electrode 210 and the second electrodes 240 may each have apillar shape, which extends in a vertical direction from the top surfaceof the substrate 200, the vertical direction corresponding to thedirection substantially perpendicular to the top surface of thesubstrate 200. In the planar view shown by FIG. 4A, the first electrode210 has a circular cross section and each of the second electrodes 240has a rectangular cross section, but the first and second electrodes 210and 240 may be shaped differently in other embodiments. The firstelectrode 210 and the second electrodes 240 may be modified to have anyof various shapes according to desired fabrication and/or operationprocesses. The first electrode 210 may face one or more of the secondelectrodes 240. That is, the first electrode 210 may be disposedadjacent to and may electrically interact with one or more of the secondelectrodes 240. As shown in the planar view of FIG. 4A, one firstelectrode 210 may face four second electrodes 240, which are spacedapart from the first electrode 210 in four directions parallel to thetop surface of the substrate 200, respectively, but the first electrode210 may face a greater or lesser number of second electrodes 240 inother embodiments. That is, the neuromorphic device may include multiplefirst electrodes like the first electrode 210, and a greater or fewernumber of the second electrodes 240. When one first electrode 210 facesmultiple second electrodes 240, the multiple second electrodes 240 maybe separated from each other.

The reactive metal layer 230 may be capable of reacting with oxygen ionsto form a dielectric oxide, and may include a metal such as Al, Ti, Ta,or Mo, or a nitride of the metal. In the cross-sectional view shown byFIG. 4B, three reactive metal layers 230 are stacked in the verticaldirection, but a number of the reactive metal layers 230 may bevariously modified as long as the stack structure ST includes multiplereactive metal layers 230. Thicknesses of the reactive metal layers 230may be substantially equal to each other. Therefore, contact areasbetween the reactive metal layers 230 and the oxygen diffusion-retardinglayer 250 may be substantially the same. In the vertical direction, theinsulating layer 235 may be interposed between two adjacent reactivemetal layers 230 and may electrically isolate the two adjacent reactivemetal layers 230 from each other. The insulating layer 235 may includeany of various insulating materials such as a silicon oxide, a siliconnitride, or a combination thereof. The stack structure ST may includetwo or more reactive metal layers 230 and the at least one insulatinglayer 235 interposed between adjacent reactive metal layers 230 in thevertical direction.

The stack structure ST may be disposed between the first electrode 210and one of the second electrodes 240. For example, in the planar viewshown by FIG. 4A, four stack structures ST may be formed, and mayrespectively correspond to the four second electrodes 240. A first sidesurface of each of the four stack structures ST may be in contact with arespective one of the four second electrodes 240, and a second sidesurface of each of the four stack structures ST, which is opposite tothe first side surface, may face the first electrode 210 in one of thefour directions. The four stack structures ST may be separated from eachother.

To electrically separate the second electrodes 240 and the stackstructures ST arranged in the four directions from each other, a spacebetween a first combination of one of the second electrodes 240 and thestack structure ST corresponding to the one second electrode 240 and asecond combination of another one of the second electrodes 240 and thestack structure ST corresponding to the other second electrode 240 maybe filled with an interlayer insulating layer 270. Furthermore, toseparate the second electrodes 240 and the stack structures ST arrangedin the four directions more reliably, a slit S may be formed in theinterlayer insulating layer 270 that is formed in the space between thefirst combination and the second combination, such that the interlayerinsulating layer 270 is divided into two portions. The slit S may beformed in a region of the interlayer insulating layer 270 disposedbetween the first combination and the second combination. For example,when the four second electrodes 240 and the four stack structures STeach have a shape extending in horizontal and longitudinal directionsfrom the first electrode 210, the slit S may have a shape extending in adiagonal direction from the first electrode 210. A filling layer 280 maybe formed in the slit S and may include an insulating material or thelike.

The oxygen-containing layer 220 may contain oxygen, and may include anyof various metal oxides, for example, an oxide of a transition metalsuch as Ti, Ni, Al, Nb, Hf, or V; a perovskite-based oxide such as PCMOor LCMO; and the like. As shown in the planar view of FIG. 4A, theoxygen-containing layer 220 is disposed against a side surface of thefirst electrode 210 and encircles the first electrode 210, but otherspatial relationships between the oxygen-containing layer 220 and thefirst electrode 210 are possible. The oxygen-containing layer 220 mayextend in a substantially vertical direction along the side surface ofthe first electrode 210, and may be interposed between the stackstructure ST and the first electrode 210. In another embodiment, theoxygen-containing layer 220 may be separated into four piecesrespectively interposed between the four stack structures ST and thefirst electrode 210. A thickness of the oxygen-containing layer 220 maybe substantially constant along a height direction that corresponds tothe direction along the side surface of the first electrode 210.However, according to desired fabrication and/or operation processes,the thickness of the oxygen-containing layer 220 may vary along theheight direction.

The oxygen diffusion-retarding layer 250 may retard the movement ofoxygen ions from the oxygen-containing layer 220 to the reactive metallayers 230. The oxygen diffusion-retarding layer 250 may be formed ofany of various semiconductor materials or dielectric materials, such asan oxide, a nitride, or a combination thereof. In the planar view shownby FIG. 4A, the oxygen diffusion-retarding layer 250 is disposed againsta side surface of the oxygen-containing layer 220 and encircles theoxygen-containing layer 220, but other spatial relationships between theoxygen diffusion-retarding layer 250 and the oxygen-containing layer 220are possible. The oxygen diffusion-retarding layer 250 may extend in asubstantially vertical direction along the side surface of theoxygen-containing layer 220, and may be interposed between the stackstructure ST and the oxygen-containing layer 220. In another embodiment,the oxygen diffusion-retarding layer 250 may be separated into fourpieces that face the four stack structures ST, respectively, and are incontact with the oxygen-containing layer 220.

The oxygen diffusion-retarding layer 250 may interfere with the movementof oxygen ions without completely blocking the movement of oxygen ions,thereby reducing the rate of formation of a dielectric oxide layer thatis formed in each of the reactive metal layers 230 at an interfacebetween each of the reactive metal layers 230 and the oxygendiffusion-retarding layer 250. That is, the oxygen diffusion-retardinglayer 250 may slow the movement of oxygen ions that migrate from theoxygen-containing layer 220 to the reactive metal layers 230. The oxygendiffusion-retarding layer 250 may have a thickness that incompletelyblocks the movement of oxygen ions. For example, it may have a thicknessof less than 10 nm.

Specially, in this embodiment, the thickness of the oxygendiffusion-retarding layer 250 may increase along a vertical directionfrom a top of the oxygen diffusion-retarding layer 250 toward a bottomof the oxygen diffusion retarding layer 250. Therefore, the thickness ofthe oxygen diffusion-retarding layer 250 between each of the differentreactive metal layers 230 having different vertical positions and theoxygen-containing layer 220 may be different. In other words, thereactive metal layers 230 having different vertical positions mayinclude dielectric oxide layers with different rates of formation and/orthicknesses at the interface with the oxygen diffusion-retarding layer250. Therefore, the symmetry and linearity of the synapse SN may beimproved. This will be described later with reference to FIGS. 5A to 5Cin more detail.

The thickest portion of the oxygen diffusion-retarding layer 250 mayhave a relatively small thickness that incompletely blocks the movementof oxygen ions.

A synapse SN may include one first electrode 210 and one of the secondelectrodes 240 facing the one first electrode 210, as well as the stackstructure ST, the oxygen diffusion-retarding layer 250, and theoxygen-containing layer 220, which are disposed between the one firstelectrode 210 and the one second electrode 240. Therefore, the planarview of FIG. 4A shows four synapses SN. These synapses SN may beseparated from each other and operate independently. However, thisembodiment is not limited thereto, and a number of the synapses SN maybe variously modified.

An example of an operating method of the synapse SN will be describedwith reference to FIGS. 5A to 5C.

FIGS. 5A to 5C are cross-sectional views illustrating a reset operationof the synapse of FIGS. 4A and 4B. FIG. 5A shows a beginning stage ofthe reset operation, FIG. 5B shows a middle stage of the resetoperation, and FIG. 5C shows a last stage of the reset operation. Forconvenience of explanation, one synapse SN is shown in FIGS. 5A to 5C.

Before the synapse SN starts to operate, that is, when the synapse SN isin an initial state, there is no dielectric oxide layer between theoxygen-containing layer 220 and the reactive metal layers 230, as shownin FIG. 4B, and thus the synapse SN may be in a low-resistance state.

If reset voltage pulses, having a certain polarity and a magnitude equalto or larger than a reset voltage, are applied to the synapse SN whenthe synapse SN is in the low-resistance state through the first andsecond electrodes 210 and 240, oxygen ions in the oxygen-containinglayer 220 may move toward the reactive metal layers 230 and may passthrough the oxygen diffusion-retarding layer 250. As a result, adielectric oxide layer may be formed in at least one of the reactivemetal layers 230 at the interface between the at least one of thereactive metal layers 230 and the oxygen diffusion-retarding layer 250.Thus, the synapse SN may undergo a reset operation, in which theresistance state of the synapse SN is changed from the low-resistancestate to a high-resistance state.

During the reset operation, a voltage applied to the second electrode240 may be greater than a voltage applied to the first electrode 210.For example, a positive voltage having a magnitude equal to or largerthan the reset voltage may be applied to the second electrode 240, and aground voltage may be applied to the first electrode 210. When the resetvoltage is applied in the form of pulses that are applied to the secondelectrode 240 repeatedly, the magnitude and width of the reset voltagepulses may be substantially constant.

In the above reset operation, referring to FIG. 5A, in the beginningstage of the reset operation, a dielectric oxide layer 260 may appearwith a small thickness in a first one of the reactive metal layers 230corresponding to the narrowest portion of the oxygen diffusion-retardinglayer 250. That is, the dielectric oxide layer 260 may be formed in theuppermost reactive metal layer 230 because the thinnest parts of theoxygen diffusion-retarding layer 250 interferes with the movement ofoxygen ions less than the thicker parts of the oxygendiffusion-retarding layer 250, and the reaction between the oxygen ionsand the reactive metal layer 230 occurs quickly in the reactive metallayer 230 adjacent to the thinnest parts of the oxygendiffusion-retarding layer 250.

Referring to FIG. 5B, as the reset operation proceeds, that is, as thenumber of the reset voltage pulses applied to the synapse SN increases,another dielectric oxide layer 260 may appear with a small thickness ina second one of the reactive metal layers 230 corresponding to a thickerportion of the oxygen diffusion-retarding layer 250. That is, in themiddle stage of the reset operation, the dielectric oxide layer 260 maybe formed in the middle reactive metal layer 230. The thickness of thedielectric oxide layer 260 that has been formed in the uppermostreactive metal layer 230 may be greater at this time than in thebeginning stage of the reset operation of FIG. 5A.

Referring to FIG. 5C, as the reset operation further proceeds, that is,the number of the reset voltage pulses applied to the synapse SN furtherincreases, a dielectric oxide layer 260 may appear with a smallthickness in a third one of the reactive metal layers 230 correspondingto the thickest portion of the oxygen diffusion-retarding layer 250.That is, in the last stage of the reset operation, the dielectric oxidelayer 260 may be formed in the lowermost reactive metal layer 230. Atthis time, the thickness of the dielectric oxide layer 260 in each ofthe uppermost and middle reactive metal layers 230 may further begreater at this time than in the middle stage of the reset operation ofFIG. 5B.

By the aforementioned method, since the rate of formation of thedielectric oxide layer 260 is reduced by the oxygen diffusion-retardinglayer 250, the reset operation may be slower than in a synapse withoutthe diffusion-retarding layer 250, and thus an abrupt change in theelectrical resistance in the beginning stage of the reset operation maybe prevented. Specifically, as the reset operation proceeds, acollective size and/or volume of the dielectric oxide layers 260 formedin the synapse SN may grow at an increasing rate. Therefore, the rate ofincrease in the electrical resistance may be relatively small in thebeginning stage of the reset operation, and the rate of increase in theelectrical resistance may grow as the reset operation proceeds. As aresult, the electrical resistance of the synapse SN does not abruptlyincrease in the beginning stage of the reset operation, and the rate ofchange in the electrical resistance during the reset operation is notsignificantly larger than the rate of change in the electricalresistance during the set operation, unlike the synapse 100 of thecomparative example. In other words, the symmetry and linearity of thesynapse SN may be secured.

Meanwhile, as described above, in the neuromorphic device of FIGS. 4Aand 4B, the number of second electrodes 240 and respective stackstructures ST that correspond to one first electrode 210, that is, thenumber of synapses SN, may be variously modified. An example of thiswill be described with reference to FIG. 6.

FIG. 6 is a planar view illustrating a neuromorphic device according toanother embodiment. A cross-sectional view taken along a line A-A′ ofFIG. 6 may be substantially the same as the cross-sectional view shownin FIG. 4B, and thus the illustration thereof will be omitted. Inaddition, differences from the above-described embodiment will mainly bedescribed.

Referring to FIG. 6, in the neuromorphic device, two second electrodes240′, and two corresponding stack structures ST′, may face one firstelectrode 210. That is, the neuromorphic device may include two synapsesSN sharing one first electrode 210.

In this regard, this embodiment may be different from the aboveembodiment of FIG. 4A. The neuromorphic device of FIG. 4A includes foursecond electrodes 240, and four corresponding stack structures ST, whichface one first electrode 210. That is, the neuromorphic deviceillustrated in FIG. 4A includes four synapses SN sharing one firstelectrode 210.

In the embodiment shown in FIG. 6, the second electrodes 240′ are spacedapart from the first electrode 210 in first and second directions. Aninterlayer insulating layer 270′ may fill a space between a firstcombination of one of the second electrodes 240′ and the stack structureST′ corresponding to the one of the second electrodes 240′ and a secondstructure of the other one of the second electrodes 240′ and the stackstructure ST′ corresponding to the other one of the second electrodes240′. That is, the interlayer insulating layer 270′ may electricallyisolate the first combination extending in the first direction from thesecond combination extending in the second direction. The firstcombination and the second combination may be disposed at opposite sidesof the first electrode 210. Furthermore, to separate the first andsecond combinations of the second electrodes 240′ and the stackstructures ST′ more reliably, a slit S′ may be formed in the interlayerinsulating layer 270′. The slit S′ may be formed along a third directionthat crosses the first and second directions, the first to thirddirections being parallel to a top surface of a substrate. For example,when the two second electrodes 240′ and the two stack structures ST′have shapes extending in two opposite horizontal directions with thefirst electrode 210, the slit S′ may have a shape extending, using thethird direction as a longitudinal direction, from the first electrode210. A filling layer 280′ may be disposed in the slit S′ and may includean insulating material, or the like.

However, in other embodiments, a number of combinations of secondelectrodes and corresponding stack structures extending from the firstelectrode may be variously modified, and cross-sectional shapes of thesecond electrodes and the corresponding stack structures may bevariously modified.

FIGS. 7A to 10B are views illustrating a method for fabricating aneuromorphic device according to an embodiment. FIGS. 7A, 8A, 9A, 9C,and 10A show planar views, and FIGS. 7B, 8B, 9B, and 10B showcross-sectional views taken along lines A-A′ of FIGS. 7A, 8A, 9A, and10A, respectively.

Referring to FIGS. 7A and 7B, an initial stack structure STa may beformed by alternately stacking a plurality of reactive metal materials231 and a plurality of insulating materials 236 over a substrate 200.

Then, four first holes H1 penetrating through the initial stackstructure STa may be formed by selectively etching the initial stackstructure STa. A second electrode material 242 having a pillar shape maybe formed in the first hole H1 by filling the first hole H1 with aconductive material. The second electrode material 242 may have a sidesurface that is in contact with each of the reactive metal materials 231in the initial stack structure STa. The position of the second electrodematerial 242 may be modified in consideration of the position of asecond electrode desired in the neuromorphic device, and additionalsecond electrode materials 242 may be added in consideration of thenumber of second electrodes desired in the neuromorphic device, whichwill be described later in further detail.

Referring to FIGS. 8A and 8B, the initial stack structure STa and thesecond electrode materials 242 may be patterned by selectively etchingthe initial stack structure STa and the second electrode materials 242.Specifically, the initial stack structure STa and the second electrodematerials 242 may be patterned into a cross-like shape, as viewed in theplanar view depicted by FIG. 8A. As a result, an intermediate stackstructure STb may be formed that has a central portion and branchportions extending from the central portion in four directions. Secondelectrodes 240 coupled to the branch portions of the intermediate stackstructure STb may also be formed. The intermediate stack structure STbmay include reactive metal layers 231′ and insulating layers 236′, whichare alternately stacked.

The process illustrated by FIGS. 8A and 8B may be used for forming theneuromorphic device of FIGS. 4A and 4B, but embodiments are not limitedto forming the neuromorphic device of FIGS. 4A and 4B. The initial stackstructure STa and the second electrode material 242 may be variouslypatterned to form various neuromorphic device structures. For example,to form the neuromorphic device of FIG. 6, two second electrodematerials penetrating through the initial stack structure STa may beformed, and the initial stack structure STa and the two second electrodematerials may be patterned into a line shape. Alternatively, when thenumber of stack structures and corresponding second electrodes is threeor more than four, the initial stack structure STa may be patterned tohave a central portion and three or more branch portions extending fromthe central portion in several directions corresponding to the number ofstack structures and corresponding second electrodes of the neuromorphicdevice being formed.

After the intermediate stack structures STb and the second electrodes240 are formed, an interlayer insulating layer 270 may be formed in aspace around the stack structures ST and the second electrodes 240 bycovering the intermediate stack structure STb and the second electrodes240 with an insulating material over the substrate 200 and performing aplanarization process until a top surface of the intermediate stackstructure STb is exposed. The planarization process may include chemicalmechanical polishing (CMP).

Referring to FIGS. 9A and 9B, a second hole H2, overlapping the centralportion of the intermediate stack structure STb and penetrating throughthe intermediate stack structure STb, may be formed by selectivelyetching the intermediate stack structure STb. The second hole H2 mayprovide a space for forming an oxygen-containing layer, an oxygendiffusion-retarding layer, and a first electrode which will be describedlater.

As shown in the planar view of FIG. 9A, when the second hole H2 has alarger width than the central portion of the intermediate stackstructure STb, the branch portions of the intermediate stack structureSTb may be completely separated from each other by the second hole H2.In this case, a process of forming a slit S may be skipped. However, forconvenience of explanation, a case in which the second hole H2 is formedtogether with the slit S is shown in FIG. 9A.

The slit S may penetrate through the interlayer insulating layer 270 andbe formed by selectively etching the interlayer insulating layer 270.The slit S may extend from the second hole H2 in a direction thatcrosses a region between the branch portions of the intermediate stackstructure STb. The process of forming the slit S may be performedsimultaneously with the process of forming the second hole H2, and theslit S and second hole H2 may be formed using the same mask and etchprocess. In an embodiment, more than one slit S may be formed in theinterlayer insulating layer 270.

As shown in the planar view of FIG. 9C, when the second hole H2 has asmaller width than the central portion of the intermediate stackstructure STb, the branch portions of the intermediate stack structureSTb may not be separated from each other. In this case, the process offorming the slit S may be performed in order to separate the branchportions from each other. The slit S may penetrate through theintermediate stack structure STb and the interlayer insulating layer 270and may be formed by selectively etching the intermediate stackstructure STb and the interlayer insulating layer 270.

Referring to FIG. 9B, as the second hole H2 is formed by selectivelyetching the intermediate stack structure STb and the interlayerinsulating layer 270, the four branch portions of the intermediate stackstructure STb are separate from each other, and thus four stackstructures ST respectively corresponding to the four branch portions areformed. The stack structure ST may include reactive metal layers 230 andinsulating layers 235, which are alternately stacked.

Referring to FIGS. 10A and 10B, an oxygen diffusion-retarding layer 250may be formed against a sidewall of the second hole H2. Here, bycontrolling processing conditions, the oxygen diffusion-retarding layer250 may be formed with a thickness that increases from the top of thediffusion-retarding layer 250 to the bottom of the diffusion-retardinglayer 250.

For example, the oxygen diffusion-retarding layer 250 may be formed byfilling the second hole H2 with an oxygen diffusion-retarding materialand dry etching the oxygen diffusion-retarding material using a maskthat exposes a central portion of the oxygen diffusion-retardingmaterial. Due to characteristics of the dry etching, a plurality ofpolymers may be accumulated on the etched surface of the etching targetas the etching proceeds, and thus the thickness of the oxygendiffusion-retarding layer 250 may increase from the top of the oxygendiffusion-retarding layer 250 to the bottom of the oxygendiffusion-retarding layer 250.

Alternatively, for example, the oxygen diffusion-retarding layer 250 maybe formed by depositing the oxygen diffusion-retarding material in athickness that does not completely fill the second hole H2 along thesidewall of the second hole H2, and performing a blanket etchingprocess. Due to characteristics of the blanket etching process, theupper portion of the etching target may be etched by a relatively largeamount, and thus the thickness of the oxygen diffusion-retarding layer250 may increase from the top of the oxygen diffusion-retarding layer250 to the bottom of the oxygen diffusion-retarding layer 250.

Meanwhile, during the deposition of the oxygen diffusion-retardingmaterial, the slit S, which has a narrow width, may also be filled withthe oxygen diffusion-retarding material. Since the slit S has a narrowwidth, the oxygen diffusion-retarding material within the slit S may notbe removed during a subsequent etching process of the oxygendiffusion-retarding material. As a result, a filling layer 280, whichmay include the oxygen diffusion-retarding material, may be formed inthe slit S. Although the slit S is not completely filled with the oxygendiffusion-retarding material, a remaining space of the slit S may befilled with an oxygen-containing material, which will be describedlater. In this case, the filling layer 280 may include the oxygendiffusion-retarding material and the oxygen-containing material.

Then, referring again to FIGS. 4A and 4B, an oxygen-containing layer 220may be formed along a sidewall of the oxygen diffusion-retarding layer250 in the second hole H2. The oxygen-containing layer 220 may be formedby depositing the oxygen-containing material along the structureillustrated by FIGS. 10A and 10B, and performing a blanket etchingprocess to the oxygen-containing material.

Then, a first electrode 210, filling a remaining space of the secondhole H2 that is partially filled with the oxygen-containing layer 220and the oxygen diffusion-retarding layer 250, may be formed. The firstelectrode 210 may be formed by depositing a conductive material in asufficient thickness to fill the second hole H2, which is partiallyfilled with the oxygen-containing layer 220 and the oxygendiffusion-retarding layer 250, and performing a planarization processuntil the top surface of the stack structure ST is exposed.

As a result, the neuromorphic device shown in FIGS. 4A and 4B may befabricated. However, the fabricating method described above may bemodified in various ways.

Meanwhile, the neuromorphic device shown in the cross-sectional views ofFIGS. 4A and 4B may be variously modified as long as the thickness ofthe oxygen diffusion-retarding layer 250 between the reactive metallayers 230 and the oxygen-containing layer 220 varies along a height ofthe oxygen diffusion-retarding layer 250. An example of this will bedescribed with reference to FIGS. 11 and 12.

FIG. 11 is a cross-sectional view illustrating a neuromorphic deviceaccording to yet another embodiment.

Referring to FIG. 11, the neuromorphic device may include a substrate300, a first electrode 310, a second electrode 340, a stack structure STdisposed between the first electrode 310 and the second electrode 340,an oxygen-containing layer 320 disposed between the stack structure STand the first electrode 310, and an oxygen diffusion-retarding layer 350disposed between the stack structure ST and the oxygen-containing layer320 The first and second electrodes 310 and 340 may be disposed over thesubstrate 300, may extend in a vertical direction perpendicular to a topsurface of the substrate 300, and may be spaced apart from each other ina horizontal direction parallel to the top surface of the substrate 300.The stack structure ST may include a plurality of reactive metal layers330 and a plurality of insulating layers 335, which are alternatelystacked over the substrate 300. The oxygen-containing layer 320 maysubstantially extend in the vertical direction along a side surface ofthe first electrode 310. The oxygen diffusion-retarding layer 350 mayalso substantially extend in the vertical direction along side surfacesof the stack structure ST and the oxygen-containing layer 320.

Here, unlike the embodiment of FIGS. 4A and 4B, a thickness of theoxygen diffusion-retarding layer 350 may decrease from a top of theoxygen diffusion-retarding layer 350 to a bottom of the oxygendiffusion-retarding layer. Since the oxygen diffusion-retarding layer350, the oxygen-containing layer 320, and the first electrode 310 areformed in a hole having a substantially vertical sidewall, the firstelectrode 310 may have a width that increases from a top of the firstelectrode 310 to a bottom of the first electrode, and may have a shapethat depends on the shape of the oxygen diffusion-retarding layer 350.The oxygen-containing layer 320 may have a substantially constantthickness along a height direction.

FIG. 12 is a cross-sectional view illustrating a neuromorphic deviceaccording to yet another embodiment.

Referring to FIG. 12, the neuromorphic device may include a substrate400, a first electrode 410, a second electrode 440, a stack structure STdisposed between the first electrode 410 and the second electrode 440,an oxygen-containing layer 420 disposed between the stack structure STand the first electrode 410, and an oxygen diffusion-retarding layer 450disposed between the stack structure ST and the oxygen-containing layer420. The first and second electrodes 410 and 440 are disposed over thesubstrate 400, extend in a vertical direction perpendicular to a topsurface of the substrate 400, and are spaced apart from each other in ahorizontal direction parallel to the top surface of the substrate 300.The stack structure ST includes a plurality of reactive metal layers 430and a plurality of insulating layers 435, which are alternately stackedover the substrate 400. The oxygen-containing layer 420 extends in thevertical direction, along the first electrode 410. The oxygen-containinglayer 450 substantially extends in the vertical direction, along a sidesurface of the stack structure ST.

Here, unlike the embodiment of FIGS. 4A and 4B, a hole, in which theoxygen diffusion-retarding layer 450, the oxygen-containing layer 420,and the first electrode 410 are disposed, may have a width thatdecreases from a top of the hole to a bottom of the hole. That is, asidewall of the hole may be sloped. A sidewall of the oxygendiffusion-retarding layer 450 may be in contact with the sloped sidewallof the hole, and another sidewall of the oxygen diffusion-retardinglayer 450 may be substantially vertical. Therefore, the thickness of theoxygen diffusion-retarding layer 450 may decrease from a top of theoxygen diffusion-retarding layer 450 to a bottom of the oxygendiffusion-retarding layer. The first electrode 410 and theoxygen-containing layer 420 may each have a substantially constant widthand/or thickness along a height direction.

In other words, the thickness of the oxygen diffusion-retarding layer450 may vary along a height direction. On the other hand, the widthand/or thickness of each of the first electrode 410 and theoxygen-containing layer 420 may or may not vary along a heightdirection.

In the above embodiments, the speed of the reset operation may begenerally reduced and the symmetry with the set operation may be ensuredby the existence of the oxygen diffusion-retarding layer. Furthermore,in the above embodiments, the resistance of the synapse may be moregradually changed due to the sequential formation and/or thicknesschange of multiple dielectric oxide layers in respective reactive metallayers, depending on the thickness of the oxygen diffusion-retardinglayer between the reactive metal layers and the oxygen-containing layer,and thus the linearity of the synapse may be ensured.

In addition, the rate of formation and/or disappearance of thedielectric oxide layer between different reactive metal layers and theoxygen-containing layer may vary depending on the contact areas and/orfacing areas between the reactive metal layers and the oxygen-containinglayer. By using the design depicted by FIG. 12, the linearity andsymmetry of the synapse may be further secured. An example of thelinearity and symmetry of the synapse will be described with referenceto FIGS. 13 and 14 in more detail.

FIG. 13 is a graph illustrating a current flowing through a synapseaccording to a size of a contact area and/or facing area between areactive metal layer and an oxygen-containing layer in the synapse.

Referring to FIG. 13, the contact area and/or facing area between thereactive metal layer and the oxygen-containing layer are proportional toa set current and a reset current of the synapse. In other words, duringa set operation, when the contact area and/or facing area between thereactive metal layer and the oxygen-containing layer is relativelylarge, the rate of disappearance of the dielectric oxide layer isrelatively high. Also, during a reset operation, when the contact areaand/or facing area between the reactive metal layer and theoxygen-containing layer is relatively large, the rate of formation ofthe dielectric oxide layer is relatively low.

FIG. 14 is a cross-sectional view illustrating a neuromorphic deviceaccording to yet another embodiment.

Referring to FIG. 14, the neuromorphic device may include a substrate600, a first electrode 610, a second electrode 640, a stack structure STdisposed between the first electrode 610 and the second electrode 640,an oxygen-containing layer 620 disposed between the stack structure STand the first electrode 610, and an oxygen diffusion-retarding layer 650disposed between the stack structure ST and the oxygen-containing layer620. The first and second electrodes 610 and 640 may be disposed overthe substrate 600, may extend in a vertical direction perpendicular to atop surface of the substrate 600, and may be spaced apart from eachother in a horizontal direction parallel to the top surface of thesubstrate 600. The stack structure ST may include a plurality ofreactive metal layers 630 and a plurality of insulating layers 635,which are alternately stacked over the substrate 600. Theoxygen-containing layer 620 may extend in the vertical direction alongthe first electrode 610. The oxygen diffusion-retarding layer 650 maysubstantially extend in the vertical direction, along a side surface ofthe stack structure ST.

Here, a thickness of the oxygen diffusion-retarding layer 650 may vary.At a given height, the thickness of the oxygen diffusion-retarding layer650 may be proportional to a height of an adjacent one of the reactivemetal layers 630. For example, the thickness of the oxygendiffusion-retarding layer 650 may decrease from a top of the oxygendiffusion-retarding layer 650 to a bottom of the oxygendiffusion-retarding layer 650, and heights of the reactive metal layers630 may respectively increase from a bottom of the stack structure ST toa top of the stack structure ST. In this case, as a reset operationproceeds, a dielectric oxide layer may be formed first within thelowermost one of the reactive metal layers 630, which has the smallestthickness or height, among the dielectric oxide layers in the synapse,while a dielectric oxide layer may be formed last in the uppermostreactive metal layer 630, which has the thinnest thickness or height,among the dielectric oxide layers in the synapse. As described above,this is because the rate of formation of each of the dielectric oxidelayers is inversely proportional to the thickness of the part of theoxygen diffusion-retarding layer 650 separating the reactive metal layer630 from the oxygen-containing layer 620.

Furthermore, thicknesses or heights of the reactive metal layers 630 maybe different from each other. For example, the thicknesses of thereactive metal layers 630 may increase along a height direction of thestack structure ST. Therefore, facing areas between different reactivemetal layers 630 and the oxygen-containing layer 620 may be differentfrom each other. Specifically, the thicknesses of the reactive metallayers 630 may be determined in consideration of the thickness of theoxygen diffusion-retarding layer 650. That is, the thickness of thereactive metal layer 630 may be relatively large when the thickness of apart of the oxygen diffusion-retarding layer 650 separating the reactivemetal layer 630 from the oxygen-containing layer 620 is relativelylarge. For example, when the thickness of the oxygen diffusion-retardinglayer 650 decreases from a top of the diffusion-retarding layer 650 to abottom of the diffusion-retarding layer 650, the thicknesses of thereactive metal layers 630 may decrease from a top of the stack structureST to a bottom of the stack ST structure. Accordingly, the facing areasbetween the reactive metal layers 630 and the oxygen-containing layer620 may decrease from the top of the stack structure ST to the bottom ofthe stack structure ST. In this case, as a reset operation proceeds, adielectric oxide layer may be formed first within the lowermost reactivemetal layer 630 while a dielectric oxide layer may be formed last in theuppermost reactive metal layer 630. The thickness of the dielectricoxide layer in the lowermost reactive metal layer 630 may be greaterthan the thickness of the dielectric oxide layer in the uppermostreactive metal layer 630. As described above, this is because the rateof formation of the dielectric oxide layer is inversely proportional tothe size of the facing area between the corresponding reactive metallayer 630 and the oxygen-containing layer 620.

As a result, by this embodiment, the rate of change in the resistancemay be reduced as the reset operation proceeds, and thus the linearityand symmetry of the synapse SN may be further secured.

The neuromorphic device according to the above-described embodiments maybe used in various devices or systems. This will be described by way ofexample with reference to FIG. 15.

FIG. 15 shows a pattern recognition system 800 according to anembodiment.

The pattern recognition system 800 may be a system for recognizingvarious patterns, such as a speech recognition system or an imagerecognition system. The pattern recognition system 800 may be configuredwith the neuromorphic device of the above-described embodiments.

Referring to FIG. 15, the pattern recognition system 800 may include acentral processing unit (CPU) 810, a memory device 820, a communicationcontrol device 830, a pattern output device 850, a pattern input device860, an analog-digital converter (ADC) 870, a neuromorphic device 880, abus line 890, and the like. The pattern recognition system 800 isconnected to a network 840 through the communication control device 830.

The central processing unit 810 may generate and transmit varioussignals used in a learning operation performed by the neuromorphicdevice 880, and may perform various processing operations forrecognizing patterns of sound, images or the like based on an output ofthe neuromorphic device 880. This central processing unit 810 may beconnected, via the bus line 890, to the memory device 820, thecommunication control device 830, the pattern output device 850, theanalog-digital converter 870, and the neuromorphic device 880.

The memory device 820 may store information in accordance withoperations of the pattern recognition system 800. For this, the memorydevice 820 may include different memory devices. For example, the memorydevice 820 may include a ROM device 822, a RAM device 824, and the like.The ROM device 822 may store various programs or data which are used inthe central processing unit 810 in order to perform the learningoperation of the neuromorphic device 880, pattern recognition, etc. TheRAM device 824 may store the program or data downloaded from the ROMdevice 822, or store data, such as sound or images, which have beenconverted or analyzed by the analog-digital converter 870.

The communication control device 830 may exchange recognized data (e.g.,sound or images) with other communication control devices through thenetwork 840.

The pattern output device 850 may output the recognized data (e.g.,sound or images) in various manners. For example, the pattern outputdevice 850 may include one or more of a printer, a display unit, and thelike, and may output sound waveforms or display images.

The pattern input device 860 may receive analog-type sound, images,etc., and may include one or more of a microphone, a camera, etc.

The analog-digital converter 870 may convert analog data, provided bythe pattern input device 860, to digital data, and may also analyze thedigital data.

The neuromorphic device 880 may perform learning, recognition, and thelike using data provided by the analog-digital converter 870, and mayoutput data corresponding to recognized patterns. The neuromorphicdevice 880 may include one or more neuromorphic devices that includesynapses of the embodiments described above. For example, theneuromorphic device 880 may include a substrate; a first electrode and asecond electrode that are disposed over the substrate, the first andsecond electrodes extending in a first direction and being spaced apartfrom each other in a second direction, the first direction beingperpendicular to a top surface of the substrate, the second directionbeing parallel to the top surface of the substrate; a stack structuredisposed between the first electrode and the second electrode, the stackstructure including a plurality of reactive metal layers which arealternately stacked with one or more insulating layers, wherein each ofthe reactive metal layers is capable of reacting with oxygen ions toform a dielectric oxide layer; an oxygen-containing layer disposedbetween the first electrode and the stack structure, theoxygen-containing layer including the oxygen ions; and an oxygendiffusion-retarding layer disposed between the stack structure and theoxygen-containing layer, the oxygen diffusion-retarding layerinterfering with a movement of the oxygen ions from theoxygen-containing layer to the reactive metal layers, and wherein athickness of the oxygen diffusion-retarding layer varies along the firstdirection. By using this configuration, the symmetry and linearity ofthe electrical conductivity of a synapse can be ensured. Accordingly,operating characteristics of the neuromorphic device 880 can beimproved, and thus operating characteristics of the pattern recognitionsystem 800 may also be improved.

The pattern recognition system 800 may further include other componentsrequired for properly performing its function(s). For example, thepattern recognition system 800 may further include one or more inputdevices such as a keyboard, a mouse, and the like, so as to receivevarious parameters and/or setting conditions for operations of thepattern recognition system 800.

According to the embodiments as described above, the symmetry andlinearity of the electrical conductivity of the synapse may be enhanced,and thus the operating characteristics of the neuromorphic device may beimproved.

While the present invention has been described with respect to thespecific embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

What is claimed is:
 1. A neuromorphic device comprising: a substrate; afirst electrode and a second electrode that are disposed over thesubstrate, the first and second electrodes extending in a firstdirection and being spaced apart from each other in a second direction,the first direction being perpendicular to a top surface of thesubstrate, the second direction being parallel to the top surface of thesubstrate; a stack structure disposed between the first electrode andthe second electrode, the stack structure including a plurality ofreactive metal layers which are alternately stacked with one or moreinsulating layers, wherein each of the reactive metal layers is capableof reacting with oxygen ions to form a dielectric oxide layer; anoxygen-containing layer disposed between the first electrode and thestack structure, the oxygen-containing layer including the oxygen ions;and an oxygen diffusion-retarding layer disposed between the stackstructure and the oxygen-containing layer, the oxygendiffusion-retarding layer interfering with a movement of the oxygen ionsfrom the oxygen-containing layer to the reactive metal layers, andwherein a thickness of the oxygen diffusion-retarding layer varies alongthe first direction.
 2. The neuromorphic device of claim 1, wherein thethickness of the oxygen diffusion-retarding layer increases from a topof the oxygen diffusion-retarding layer to a bottom of the oxygendiffusion-retarding layer.
 3. The neuromorphic device of claim 2,wherein thicknesses of the plurality of reactive metal layers increasealong a direction from a top of the stack structure to a bottom of thestack structure.
 4. The neuromorphic device of claim 1, wherein thethickness of the oxygen diffusion-retarding layer decreases from a topof the oxygen diffusion-retarding layer to a bottom of the oxygendiffusion-retarding layer.
 5. The neuromorphic device of claim 4,wherein thicknesses of the reactive metal layers decrease along adirection from a top of the stack structure to a bottom of the stackstructure.
 6. The neuromorphic device of claim 1, wherein the secondelectrode includes a plurality of second electrodes, which are spacedapart from each other, and wherein the stack structure includes aplurality of stack structures, which are spaced apart from each other,each of the plurality of stack structures corresponding to a respectiveone of the plurality of the second electrodes.
 7. The neuromorphicdevice of claim 6, further comprising: an interlayer insulating layerdisposed between two adjacent second electrodes and between two adjacentstack structures corresponding to the two adjacent second electrodes. 8.The neuromorphic device of claim 7, further comprising: a slitpenetrating through the interlayer insulating layer and extending in adirection crossing a region between the two adjacent second electrodesand between the two adjacent stack structures, the slit extending fromthe first electrode.
 9. The neuromorphic device of claim 8, furthercomprising: a filling layer disposed in the slit, the filling layerincluding the same material as at least one of the oxygen-containinglayer and the oxygen diffusion-retarding layer.
 10. The neuromorphicdevice of claim 1, wherein thicknesses of the plurality of reactivemetal layers are different from each other.
 11. The neuromorphic deviceof claim 1, wherein the oxygen-containing layer is disposed against aside surface of the first electrode, the oxygen-containing layerencircling the first electrode.
 12. The neuromorphic device of claim 11,wherein the oxygen diffusion-retarding layer is disposed against a sidesurface of the oxygen-containing layer, the oxygen diffusion-retardinglayer encircling the oxygen-containing layer.
 13. The neuromorphicdevice of claim 1, wherein a reset operation is performed when a resetvoltage having a first polarity is applied through the first electrodeand the second electrode, the reset operation including forming orthickening the dielectric oxide layer in at least one of the pluralityof reactive metal layers at an interface with the oxygendiffusion-retarding layer, and wherein a set operation is performed whena set voltage having a second polarity opposite to the first polarity isapplied through the first electrode and the second electrode, the setoperation including disappearing or thinning the dielectric oxide layer.14. The neuromorphic device of claim 13, wherein the dielectric oxidelayer is formed the fastest in a first reactive metal layer of theplurality of reactive metal layers, the first reactive metal layer beingseparated from the oxygen-containing layer by a thinnest region of theoxygen diffusion-retarding layer.
 15. The neuromorphic device of claim13, wherein thicknesses of the plurality of reactive metal layers aredifferent from each other, and the dielectric oxide layer is formed thefastest in a first reactive metal layer of the plurality of reactivemetal layers, the first reactive metal layer having the smallestthickness of the plurality of reactive metal layers.
 16. Theneuromorphic device of claim 13, wherein the dielectric oxide layerincludes one or more dielectric oxide layers, and wherein an electricalconductivity decreases as a number of the one or more dielectric oxidelayers increases or a thickness of each the one or more dielectric oxidelayers increases, and the electrical conductivity increases as thenumber of the one or more dielectric oxide layers decreases or thethickness of each of the one or more dielectric oxide layers decreases.17. The neuromorphic device of claim 13, wherein the dielectric oxidelayer includes one or more dielectric oxide layers, and wherein, duringthe reset operation, a number of the one or more dielectric oxide layersincreases or a thickness of each of the one or more dielectric oxidelayers increases when a number of pulses of the reset voltage appliedthrough the first electrode and the second electrode increases, andwherein, during the set operation, the number of the one or moredielectric oxide layers decreases or a thickness of each of the one ormore dielectric oxide layers decreases when a number of pulses of theset voltage applied through the first electrode and the second electrodeincreases.
 18. The neuromorphic device of claim 17, wherein, during thereset operation, the pulses of the reset voltage have a constant widthand magnitude, and wherein, during the set operation, the pulses of theset voltage have a constant width and magnitude.
 19. The neuromorphicdevice of claim 1, wherein the oxygen diffusion-retarding layer has athickness that incompletely blocks the movement of the oxygen ions. 20.The neuromorphic device of claim 1, wherein the oxygendiffusion-retarding layer comprises a dielectric material, asemiconductor material, or a combination of the dielectric material andthe semiconductor material.